This invention relates to a method of diagnosing mammalian pathology or target tissues and more particularly to a photoidentification method of diagnosing pathology or target tissue using an optical imaging material containing an imaging agent and at least one auxiliary chromophore. The imaging material preferentially localizes in pathology or target tissue, absorbs light and in some cases fluoresces or phosphoresces upon exposure to light. The primary purpose of the auxiliary chromophore is prevention of photodamage to healthy tissue by the agent.
Certain classes of molecules, including, for example, synthetic porphyrin derivatives, naturally occurring porphyrins and their derivatives, chlorophylls and their derivatives, purpurins, phthalocyanines, other cyclic tetrapyrroles, and fullerenes can act as imaging, detection and diagnostic agents for pathologies or target tissues including tumors, atherosclerotic and arthritic tissue, and diseased blood vessels. Administration of these agents to a human or other organism results in preferential localization of the agent in any of a variety of pathologies with respect to surrounding tissue. Irradiation of the organism with light of a given wavelength or wavelengths results in absorption of light by the agent. In some cases, the agent then emits light by fluorescence or phosphorescence. Light absorption or light emission produces contrast between the pathology or target tissue and the surrounding tissue, and the detection of this contrast allows pathology or target tissue imaging, detection or diagnosis. Alternatively, agents of this type can be used to enhance contrast or otherwise improve detection in magnetic resonance imaging of pathology or target tissues or can bear radioactive isotopes whose detection can be the basis of pathology or target tissue imaging, detection and diagnosis, or can serve as contrast agents for X-ray radiological or other techniques involving high-energy radiation.
Absorption of light by the agent results in production of excited states. These excited states are by definition of higher energy than the original unexcited ground state of the agent. The excess energy can result in deleterious interactions with the organism. For example, excited triplet states of the agent (or singlet or other excited states) can react directly with tissue or other components of the organism to cause damage to the organism. Triplet states and other states of high multiplicity can also cause the formation of excited states of oxygen, such as singlet oxygen, and other powerful oxidizing agents, superoxides, and other oxygen radicals. These excited states of oxygen and oxygen radicals are known to cause damage to biological membranes as well as other components of the organism. The agent in an excited state can also react with other molecules present to create other species that are harmful to the organism. Such damage is not limited to pathology or target tissue, as the agent does not localize exclusively in the pathology or target tissue. After administration some agent is found throughout the organism, including the skin. This propensity to cause damage to healthy tissue in the organism can limit the usefulness of the agent for pathology or target tissue imaging detection and diagnosis.
Carotenoid pigments, which are ubiquitous in photosynthetic membranes, are essential for the survival of green plants. Three facets of carotenoid function are recognized in photosynthetic membranes. First, carotenoids photoprotect by rapidly quenching chlorophyll triplet states which are formed in antenna systems or photosynthetic reaction centers. This triplet-triplet energy transfer prevents chlorophyll-photosensitized formation of highly destructive singlet oxygen which is injurious to the organism. In addition, carotenoids act as antennas by absorbing light in spectral regions where chlorophyll absorbs weakly and by delivering the resulting excitation to chlorophyll via a singlet-singlet energy transfer process. Finally, nearby carotenoids quench chlorophyll first excited singlet states. This quenching has been ascribed to energy transfer or electron transfer or some other process leading to internal conversion and is believed to play a role in the regulation of photosynthesis.
A number of porphyrin materials have been found to localize in pathologies and damage that tissue upon irradiation with light. Many of these, such as "hematoporphyrin derivative" and related materials, are being investigated as photodynamic therapeutic agents. All of these agents suffer from the problem that they are also absorbed by healthy tissue, which is consequently harmed by light.
Various synthetic carotenoids designed to mimic carotenoid photo protection have been investigated. Synthetic carotenoporphyrins consisting of a carotenoid part covalently linked to a synthetic meso-tetraarylporphyrin which successfully exhibited the photophysical functions of cartenoids in photosynthesis were first reported by G. Dirks, A. Moore, T. Moore and D. Gust in Photochemistry and Photobiology, Vol. 32, pp. 277-280 (Permagon Press Ltd. Great Britain, 1980).
A carotenoporphyrin which demonstrated quenching of the porphyrin triplet state by the attached carotenoid via triplet-triplet energy transfer was reported by R. V. Bensasson, E. J. Land, A. L. Moore, R L. Crouch, G. Dirks, T. A. Moore and D. Gust in Nature, Vol. 290, No. 5804, pp. 329-332 (Mar. 16, 1981). Since that time, various compounds which exhibit such triplet-triplet energy transfer have been reported. In 1984, five carotenoporphyrins were prepared by Dr. Paul Liddell at Arizona State University and reported in his doctoral thesis dated December 1985. Three carotenoporphyrins were reported by H. Frank, B. Chadwick, J. Oh, and D. Gust et al. in Biochemical et Biophysical Acta 892 (1987), pp. 253-263.
It has also been previously shown in U.S. Pat. No. 5,286,474, incorporated by reference herein, that certain synthetic carotenoporphyrins preferentially localize in mammalian pathology or target tissue where they absorb and emit light when irradiated with light so that the site of the pathology or target tissue may be detected by the fluorescence of the localized carotenoporphyrin.